Abstract: A probe for detecting separated sludges such as in sewage, is axially elongated and has a large number of LED-phototransistor pairs axially spaced thereon in facing relationship with a gap therebetween. The phototransistors are commonly connected, and the LEDs are sequentially fired by multiplex switching located within the probe. Since the quiescent current of the phototransistors is normally low, the common phototransistor output will essentially be proportional to the amount of light reaching a specific phototransistor. The output signals are processed to automatically locate an interface between separated sludge and clarified liquor. Circuitry is included to adjust the power level at which the LEDs are activated responsive to the output from selected ones of the phototransistors, for the purpose of reducing beam intensity if the sludge becomes thin, and also for increasing the intensity to overcome fouling of the windows of the probe.